That would burn bandwidth and loading time, since the only way to check the hash is to download the content. Part of the point of ad blockers is to stop the re

That would burn bandwidth and loading time, since the only way to check the hash is to download the content. Part of the point of ad blockers is to stop the request for the ad as early as possible. Regardless, it's easy enough to salt the ads for each request, thwarting hash checks.